Dental flask

ABSTRACT

The closure section of a two-section dental flask, used in the manufacture of dentures, fulcrums to a limited, adjustable extent about a bevel on the wall of the primary section of the flask during curing of polymeric material in the base of a denture contained within the flask. By establishing the fulcrum point behind the molar tooth region of the denture, expansion of polymeric material in the molar region may be greatly restricted, while the anterior tooth region, having less material than the molar region, is essentially free to expand and contract normally under pressure of a spring action compress. Connector pins for the sections of the flask fit into double-diameter holes to facilitate rocking and fulcruming of the closure section while preventing twisting thereof, and a rim on the outside of the closure section receives and transmits pressure from the compress directly to the pyramidal sidewall of the primary section. Special breakers and a pad on the inside of the closure section cooperate to safely dislodge the finished denture from solidified investment material in the primary section when the solidified material is ejected from the primary section.

United States Patent 1 McGowan Nov, 2 11, 11973 [22] Filed:

[ DENTAL FLASK [76] Inventor: George F. McGowan, 4509 Forest,

Kansas City, M0. 641 10 Nov. 8, 19711 [21] Appl. No; 196,632

Related ILLS. Application Data [63] Continuation-impart of Ser. No. 31,947, April 27,

1970, abandoned.

[52] US. Cl. 32/59 [51] Int. Cl. A61e 3/06 [58] Field of Search 32/2; 164/376, DIG. 4,

[56] Retell-enemas Cited UNITED STATES PATENTS 2,660,758 12/1953 Hennike et al. 164/376 Primary Examiner-Louis G. Mancene Assistant ExaminerJ. Q. Lever Attorney-Schmidt, Johnson, Hovey & Williams ABSTRACT The closure section of a two-section dental flask, used in the manufacture of dentures, fulcrums to a limited, adjustable extent about a bevel on the wall of the primary section of the flask during curing of polymeric material in the base of a denture contained within the flask. By establishing the fulcrum point behind the molar tooth region of the denture, expansion of polymeric material in the molar region may be greatly restricted, while the anterior tooth region, having less material than the molar region, is essentially free to expand and contract normally under pressure of a spring action compress. Connector pins for the sections of the flask fit into double-diameter holes to facilitate rocking and fulcruming of the closure section while preventing twisting thereof, and a rim on the outside of the closure section receives and transmits pressure from the compress directly to the pyramidal sidewall of the primary section. Special breakers and a pad on the inside of the closure section cooperate to safely dislodge the finished denture from solidified investment material in the primary section when the so lidified material is ejected from the primary section.

17 Qllnims, 6 Drawing Figures DENTAL FLASK CROSS REFERENCES This is a continuation-in-part of my co-pending U.S. Pat. application titled DENTAL FLASK, Ser. No.

31,947, filed Apr. 27, 1970, now abandoned.

This invention relates to improvements in dental flasks used in the manufacture of dental appliances. The steps of the various techniques of prosthetic denistry are well known to those skilled in the art and are outlined in my U. S. Pat. Nos. 2,899,707 and 2,975,479; accordingly, such techniques will not hereinafter be described in detail. While the dental flasks disclosed in the above patents have proven to be highly satisfactory, certain improved features are presented by the dental flask of the present invention which contribute substantially to attainment of the goal of providing a dental flask which, when properly used, assures the manufacturer of a well fitting dental appliance of high quality conforming accurately to the positive model upon which the appliance is based.

One important object of the present invention is to provide a two-section dental flask having pin-like connectors that prevent the sections from twisting relative to each other, yet permit one of the sections to be rocked relative to the other section so that the flask may be easily disassembled.

Another important object of the instant invention is to provide a dental flask which, when closed or clamped by a compress, insures that pressure applied to the flask is transmitted entirely through the sidewalls thereof.

A further important object of the invention is the provision of pyramidal sidewall construction for the primary denture-containing section of the flask which minimizes the possibility of distortion of the sidewall under outward pressure exerted from within such section.

An additional important object of this invention is to provide means for preventing excessive expansion of the denture in the molar tooth region thereof during polymerization of the denture base material while the flask is held in a compress, thus avoiding the highly unsatisfactory and frustrating situation wherein the molar region of the finished denture, while having been very carefully prepared with close adherance to the positive pattern for the denture, nonetheless is disproportionately larger than the anterior tooth region thereof because of differences in heating and cooling factors in the two regions.

Still further important objects of the present invention are to provide a flask which facilitates ejection of investment material containing the finished denture from the flask without causing damage to the denture itself, and to provide special structure for safely breaking the investment material away from the denture during the ejection process, thereby obviating the need for striking the flask with a damaging blow from a hammer or similar tool to free the investment material and the denture.

In the drawing:

FIG. I is a perspective view of the primary, intermediate section of a dental flask constructed in accordance with the principles of the present invention;

FIG. 2 is a perspective view of the underside of a closure section of the flask;

FIG. 3 is a perspective view of the top side of the closure section of FIG. 2;

FIG. 4 is a top plan view of the ejector section of the flask;

FIG. 5 is a vertical cross-sectional view of the flask, showing the latter within a compress; and

FIG. 6 is an enlarged, fragmentary vertical crosssectional view of the flask detailing one of the alignment connectos for the primary and closure sections of the flask.

The improved dental flask includes a substantially tubular primary section 10 (FIG. 1) having a continuous sidewall 12, a chamber 14, and a pair of open, lower and upper ends 16 and 18 respectively. The sidewall 12 is of pyramidal configuration, top to bottom, presenting an inwardly sloping inner surface and an outwardly sloping outer surface 12b, and a bridge 20 extends across the lower end 16 and is provided with a heat conductor 22 in the form of a boss formed integrally with the bridge 20. A pair of cars 24 are formed at a pair of adjacent front comers of the sidewall 12, and a pin 26 extends from each ear 24 at the upper end 18. A rectangular block 28 at the back of section 10 proj ects outwardly from sidewall 12 and provides an additional area at the top of sidewall 12 for a threaded hole 30. A bevel 32, beginning at hole 30 and extending across the full width of the back of section 10, slopes downwardly away from the top of sidewall 12 and chamber 14 until the outer surface ll2b is reached.

The flask further includes a closure section 34 (FIGS. 2 and 3) adapted to cover end 18 of section 10 as illustrated in FIGS. 5 and 6. Section 34 has a recessed inner face 36 provided with a peripheral edge 38 adapted to engage the top of upper end 18 of sidewall 12. The recessed nature of face 36 presents a cavity 40 within which are disposed a pair of opposed, blade-like projections 42, each integral with section 34. A dishshaped, ramp-like pad 44 is strategically located in cavity 40 at the back of the latter and presents an arcuate, inclined surface (see particularly FIG. 5). A block 46, similar to block 28 on section 10, is integral with the back of closure section 34 and has a smooth, counterbored hole 48 disposed proximal thereto completely through section 34 for alignment with threaded hole 30 in section 10 when the flask is assembled. A hold-down bolt element 50 (FIG. 5) is loosely received by hole 48 and threadably received by hole 30 when the flask is assembled.

Section 34 is also provided with a recessed outer face 52 presenting a raised rim edge 54 adjacent the periphery of section 34 and integral therewith. A pair of flanges 56 project laterally from opposite sides of the closure section 34 and have outermost edges which present a pair of finger-tip grips.

A pair of bores extend through section 34, on opposite front comers of the latter, each bore being adapted to receive a corresponding pin 26 when the flask is assembled with section 34 closing end 18 of section 110. Each bore includes an elongated opening 53 of substantially the same diameter as the corresponding pin 26 and an elongated aperture 60 remote from the primary section 10 in alignment with the opening 58 and of substantially larger diameter than the pin 26. Each opening 58 extends from the edge 38 of section 34 a substantial distance toward rim 54 with the corresponding aperture 60 beginning where opening 58 terminates and extending the remainder of the distance to rim 54. A pair of recessed shelves 62 extend transversely of corresponding pins 26 and are formed in edge 38 adjacent the openings 58.

An ejector section 64 shown in detail in FIG. 4 has a flat, planar portion 66, a pair of upstanding, curved guides 68 at opposite back comers of portion 66, and a single, upstanding, elongated guide 70 across the front of portion 66. As shown in FIG. 5, guides 68 and 70 serve to embrace the lower part of section 10 when assembled sections 34 and 10 are placed on ejector section 64. A pair of strategically located, generally elliptical protuberances 72 rise from portion 66, each being provided with an inclined surface 74 facing upwardly into chamber 14 of section 10 and aligned with pad 44 as shown in FIG. and an upwardly facing surface 76 which is substantially parallel to ends 16 and 18 and is disposed forwardly of pad 44.

In use, the steps in preparation of a denture as outlined in my U. S. Pat. Nos. 2,899,707 and 2,975,479 are easily and most effectively carried out. After a pattern has been formed for the denture within investment material disposed in the primary section and in the closure section 34, the artificial teeth which have been previously mounted upon a wax base remain embedded within the investment material of closure section 34 when the latter and section 10 are separated. Thereupon, a suitable polymeric substance is deposited within the pattern previously formed by the now removed wax base, and the sections 10 and 34 are reassembled and placed between opposed jaws of a suitable curing compress, such as the type disclosed in my U. S. Pat. Nos. 3,267,525; 3,4l 1,184; and 3,571,858.

A representative compress of this type is illustrated in FIG. 5 having a pair of opposed pressure plates 78 which are actuated by a central, threaded member 80, it being understood that the precedure illustrated in FIG. 5 in reality relates to the later step of ejecting the finished denture from the flask. Preferably, the curing compress is of a spring-action type.

As the polymeric substance cures within the confines of the solidified investment material within the flask, the substance seeks to expand; in this respect, because of the fact that the substantially more polymeric substance is required for the posterior, molar tooth region of the denture than the anterior tooth region thereof, the molar region seeks to expand to a much greater extent. For purposes of illustration, a denture designated by the numeral 82 is shown in FIG. 5 with the anterior tooth region thereof designated 82a and the molar tooth region thereof designated 82b, the entire denture 82 being encompassed by solidified investment material 84. As the polymeric substance seeks to expand against the yieldable action of the spring compress, the bolt 50 and bevel 32 on section 10 come into play. Depending upon the extent to which bolt 50 has been tightened down, closure section 34 will begin to fulcrum about an axis passing through bolt 50, with the junction of bevel 32 with the top surface of end 18 defining the fulcrum and its axis.

Because the fulcrum point for section 34 is disposed rearwardly of the molar region 82!), the front of section 34 is permitted to raise more than the back thereof, effecting a like amount of restraint on the expanding polymeric substance. Accordingly, the molar region 82b is restricted from expanding to a much greater extent than is the anterior tooth region 820, which normally contains substantially less material. Therefore, when the polymeric substance is cooled, after expanding, it contracts to a position wherein the molar region 82b is not disproportionately larger than anterior region 82a. at variance with the accurately and carefully prepared pattern for the denture. Without the limited fulcruming action of section 34, it is quite likely that the finished denture 82 may have too large a molar region 82b as compared to anterior region 82a such that a proper bight in the latter region may not be obtained. This is possible even in spite of the fact that the other steps for preparing denture 82 have been very carefully and strictly adhered to prior to the curing process.

After the polymeric substance of denture 82 has been cured, the flask is disassembled and the investment material within cavity 40 of section 34 removed. During separation of the two sections 10 and 34, it will be ap preciated that a suitable tool such as a screwdriver may be inserted beneath either of the recessed shelves 62 of section 34 to aid in prying the flask apart. It will be appreciated that the multi-diameter bores through the front corners of section 34 facilitate rocking of the latter during this time, yet prevent any twisting motion transversely of the pins 26 which could damage the denture 82.

After the investment material has been removed from cavity 40, the flask is again reassembled and placed between the pressure plates 78 with the addition of ejector section 64 as illustrated in FIG. 5. Pressure is then directed to the flask via the rim 54 on the top of section 34 from plates 78 as member 80 is rotated. Such localized transmittal of the pressure is extremely important during the earlier stages of manufacture of denture 82 to prevent the application of forces directly to the investment material and the denture 82, resulting in deformation of the latter. Moreover, the pyramidal configuration of sidewall 12 assures that deformation is greatly resisted to preclude any distortion of the denture 82.

As pressure is again applied to the flask, the protuberances 72 on ejector section 64 move progressively into the bottom of chamber 14 to engage investment material 84 and force the latter up into the empty cavity 40 in section 34. During such ejection of material 84 from chamber 14 the pad 44 which overlies the investment material 84 above molar region 82b engages material 84 to restrict and stabilize its movement. The anterior region 82a, however, is free to move into cavity 40 without restraint, thus preventing damage to the fragile anterior teeth of region 82a. During this time, the inclined surfaces 74 of protuberances 72 prevent an excessive build up of pressure in molar region 82b since inclined surfaces 74 underlie the latter in alignment therewith.

As ejection continues, the breaker projections 42 which overlie investment material 84 and denture 82 engage and fracture material 84 to release the finished denture 82 without damaging the latter. Thereupon, the sections 10 and 34 may be disassembled and the finished denture 82 removed.

By providing for simple and quick removal of investment material 84 and denture 82 in this manner, it is no longer necessary to beat on section 10 with a hammer or other instrument in order to remove material 84 and release denture 82 therefrom. Accordingly, any damage to section 10 is totally obviated.

Moreover, because of the unique fulcruming action of the closure section 34 of the flask during curing of the denture base substance, it is now possible to produce a finished denture corresponding precisely to the pattern from which it is prepared. Any possibility that the molar region teeth will project above or below their intended level because of excessive expansion of the denture base in the molar region is virtually eliminated. Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. In a dental flask: a substantially tubular, primary section having an open end; a plurality of pins extending from said end; and a closure section disposed in closing relationship with said end and having a plurality of bores therein receiving corresponding pins, each bore including an elongated opening of substantially the same diameter as the corresponding pin to preclude relative movement of the sections transversely of said pins, each bore further including an elongated aperture registering and aligned with a corresponding opening remote from the primary section, each of said apertures receiving and being substantially larger in diameter than that portion of the corresponding pin disposed within the aperture, whereby the closure section may be rocked on the primary section to facilitate separation of the sections. 2. The invention of claim 1, said closure section having an inner edge adjacent said end of the primary section and an outer edge spaced from the latter, each opening extending longitudinally of its bore from said inner edge toward said outer edge and having a terminus intermediate said edges, each aperture extending longitudinally of its bore from said terminus of the corresponding opening to said outer edge. 3. The invention of claim 2, each opening extending a substantial part of the distance between said inner and outer edges of the closure section. 4. In a dental flask: a substantially tubular, primary section provided with a pair of opposed, open ends, said primary section having a chamber therein for receiving a dental appliance and investment material in surrounding relationship thereto; a closure section disposed in closing relationship with one end of said primary section, and having a cav-- ity therein communicating with said chamber;

means receivable within the opposite end of said primary section for engagement with the investment material in said chamber to force said material toward said one end and into said cavity;

a rigid projection overlying said chamber in the path of travel of said material for fracturing the material as the latter is forced into the cavity, whereby to free the dental appliance from said material for removal therefrom; and

a pad on saidclosure section in said cavity having an inclined surface leading from a lateral extremity of said cavity into the latter and facing said chamber for stabilizing movement of said material as the latter is forced into the cavity into engagement with the pad and fractured by said projection.

5. In a dental flask:

a substantially tubular, primary section provided with a pair of opposed, open ends,

said primary section having a chamber therein for receiving a dental appliance and investment material in surrounding relationship thereto;

a closure section disposed in closing relationship with one end of said primary section, and having a cavity therein communicating with said chamber;

ejector means receivable within the opposite end of said primary section for engagement with the in vestment material in said chamber to force said material toward said one end and into said cavity; and

a pad extension integral with said closure section and having an inclined surface leading from a lateral extremity of said cavity into the latter and facing said chamber to stabilize movement of the material as the latter is forced into the cavity and into engagement with the surface, whereby to prevent damage to the dental appliance within said material as the latter is ejected from said chamber.

6. The invention of claim 5,

said ejector means including a protuberance having a pair of surfaces facing said cavity, one substantially parallel with said open ends and the other inclined, said one surface being aligned with one portion of said appliance, the other surface being aligned with a second portion of said appliance for relieving pressure on said second portion as said material is forced into said cavity against said pad.

7. The invention of claim 5,

said pad being aligned with and spaced above the posterior portion of said dental appliance to restrict movement of posterior teeth and surrounding investment material and permit substantial, relatively free movement of anterior teeth into said cavity to minimize the likelihood of cracking the anterior teeth.

8. The invention of claim 7,

said ejector means including a protuberance having a pair of surfaces facing said cavity, one substantially parallel with said open ends and the other inclined, said one surface being aligned with the anterior portion of said appliance, the other surface being aligned with the posterior portion of said appliance and with said inclined surface of the pad for relieving pressure on posterior teeth as said material is forced into said cavity against said pad.

9. In combination with the pressure plate of a compress, a dental flask including:

a substantially tubular, primary section having an investment-retaining sidewall and an open end;

a closure section having an outer face spanning said sidewall in closing relationship to said end; and

a raised rim on said face adjacent the periphery of the closure section and in pressure engagement with said plate to direct the applied force exclusively to said sidewall.

10. The invention of claim 9,

said rim being integral with the closure section,

said outer face having a central, recessed portion presenting said rim.

111. The invention of claim 10,

said closure section being provided with a pair of flanges projecting from opposite sides thereof and presenting a pair of finger-tip grips.

12. The invention of claim lll,

each of said flanges having an outermost, finger-tipengaging edge.

13. In a dental flask:

a substantially tubular flask section provided with a pair of opposed ends and having a chamber therein for receiving a dental appliance and investment material in surrounding relationship thereto; and

means closing said ends of the section and thereby closing said chamber,

said section having a peripheral sidewall defining said chamber and extending between said ends,

said sidewall being truncatedly pyramidal in crosssection, presenting a pair of opposed, inner and outer surfaces extending between said ends and sloping inwardly and outwardly, respectively, with respect to said chamber,

said closing means including a closure section spanning said sidewall at said one end thereof, and having an outer face adapted to receive force applied thereto by a pressure plate of a compress,

said sidewall surfaces diverging from the direction of force transmitted to the sidewall by said closure section as said surfaces extend from said one end toward said other end,

said closure section being provided with a raised rim on said face adjacent the periphery of the closure section,

said rim being adapted for engagement by said pressure plate to transmit the force exclusively to said sidewall.

14. In a dental flask:

a substantially tubular, primary section provided with an open end, said primary section having a chamber therein for retaining investment material having the pattern of a dental appliance formed therein,

said pattern being adapted to receive a deposit of curable substance for forming, upon curing of said substance, an appliance corresponding in size and shape to said pattern and having an anterior and a posterior region;

a closure section disposed in closing relationship with said open end of the primary section and having a cavity therein for retaining investment material which cooperates with said material in the chamber of the primary section to surround the appliance during formation of the latter,

said open end of the primary section having a bevel presenting a fulcrum for said closure section; and

hold-down means interconnecting said sections at said bevel to permit limited fulcruming of the closure section relative to the primary section about the axis of the fulcrum disposed rearwardly of said posterior region of the appliance to thereby restrict expansion of said curable substance in the posterior region to a greater extent than in the anterior region during curing of the substance.

15. The invention of claim 14,

said hold-down means including an element extending from said closure section into said primary section and threadably received by the latter.

16. The invention of claim 15, said axis intersecting said element.

17. The invention of claim 14,

said primary section having a sidewall defining the limits of said chamber and said open end defining the top of said sidewall,

said bevel being disposed at said top of the sidewall and sloping downwardly therefrom to the outer margin of the sidewall. 

1. In a dental flask: a substantially tubular, primary section hAving an open end; a plurality of pins extending from said end; and a closure section disposed in closing relationship with said end and having a plurality of bores therein receiving corresponding pins, each bore including an elongated opening of substantially the same diameter as the corresponding pin to preclude relative movement of the sections transversely of said pins, each bore further including an elongated aperture registering and aligned with a corresponding opening remote from the primary section, each of said apertures receiving and being substantially larger in diameter than that portion of the corresponding pin disposed within the aperture, whereby the closure section may be rocked on the primary section to facilitate separation of the sections.
 2. The invention of claim 1, said closure section having an inner edge adjacent said end of the primary section and an outer edge spaced from the latter, each opening extending longitudinally of its bore from said inner edge toward said outer edge and having a terminus intermediate said edges, each aperture extending longitudinally of its bore from said terminus of the corresponding opening to said outer edge.
 3. The invention of claim 2, each opening extending a substantial part of the distance between said inner and outer edges of the closure section.
 4. In a dental flask: a substantially tubular, primary section provided with a pair of opposed, open ends, said primary section having a chamber therein for receiving a dental appliance and investment material in surrounding relationship thereto; a closure section disposed in closing relationship with one end of said primary section, and having a cavity therein communicating with said chamber; means receivable within the opposite end of said primary section for engagement with the investment material in said chamber to force said material toward said one end and into said cavity; a rigid projection overlying said chamber in the path of travel of said material for fracturing the material as the latter is forced into the cavity, whereby to free the dental appliance from said material for removal therefrom; and a pad on said closure section in said cavity having an inclined surface leading from a lateral extremity of said cavity into the latter and facing said chamber for stabilizing movement of said material as the latter is forced into the cavity into engagement with the pad and fractured by said projection.
 5. In a dental flask: a substantially tubular, primary section provided with a pair of opposed, open ends, said primary section having a chamber therein for receiving a dental appliance and investment material in surrounding relationship thereto; a closure section disposed in closing relationship with one end of said primary section, and having a cavity therein communicating with said chamber; ejector means receivable within the opposite end of said primary section for engagement with the investment material in said chamber to force said material toward said one end and into said cavity; and a pad extension integral with said closure section and having an inclined surface leading from a lateral extremity of said cavity into the latter and facing said chamber to stabilize movement of the material as the latter is forced into the cavity and into engagement with the surface, whereby to prevent damage to the dental appliance within said material as the latter is ejected from said chamber.
 6. The invention of claim 5, said ejector means including a protuberance having a pair of surfaces facing said cavity, one substantially parallel with said open ends and the other inclined, said one surface being aligned with one portion of said appliance, the other surface being aligned with a second portion of said appliance for relieving pressure on said second portion as said material is forced into said cavity against said pad.
 7. The invention of claim 5, sAid pad being aligned with and spaced above the posterior portion of said dental appliance to restrict movement of posterior teeth and surrounding investment material and permit substantial, relatively free movement of anterior teeth into said cavity to minimize the likelihood of cracking the anterior teeth.
 8. The invention of claim 7, said ejector means including a protuberance having a pair of surfaces facing said cavity, one substantially parallel with said open ends and the other inclined, said one surface being aligned with the anterior portion of said appliance, the other surface being aligned with the posterior portion of said appliance and with said inclined surface of the pad for relieving pressure on posterior teeth as said material is forced into said cavity against said pad.
 9. In combination with the pressure plate of a compress, a dental flask including: a substantially tubular, primary section having an investment-retaining sidewall and an open end; a closure section having an outer face spanning said sidewall in closing relationship to said end; and a raised rim on said face adjacent the periphery of the closure section and in pressure engagement with said plate to direct the applied force exclusively to said sidewall.
 10. The invention of claim 9, said rim being integral with the closure section, said outer face having a central, recessed portion presenting said rim.
 11. The invention of claim 10, said closure section being provided with a pair of flanges projecting from opposite sides thereof and presenting a pair of finger-tip grips.
 12. The invention of claim 11, each of said flanges having an outermost, finger-tip-engaging edge.
 13. In a dental flask: a substantially tubular flask section provided with a pair of opposed ends and having a chamber therein for receiving a dental appliance and investment material in surrounding relationship thereto; and means closing said ends of the section and thereby closing said chamber, said section having a peripheral sidewall defining said chamber and extending between said ends, said sidewall being truncatedly pyramidal in cross-section, presenting a pair of opposed, inner and outer surfaces extending between said ends and sloping inwardly and outwardly, respectively, with respect to said chamber, said closing means including a closure section spanning said sidewall at said one end thereof, and having an outer face adapted to receive force applied thereto by a pressure plate of a compress, said sidewall surfaces diverging from the direction of force transmitted to the sidewall by said closure section as said surfaces extend from said one end toward said other end, said closure section being provided with a raised rim on said face adjacent the periphery of the closure section, said rim being adapted for engagement by said pressure plate to transmit the force exclusively to said sidewall.
 14. In a dental flask: a substantially tubular, primary section provided with an open end, said primary section having a chamber therein for retaining investment material having the pattern of a dental appliance formed therein, said pattern being adapted to receive a deposit of curable substance for forming, upon curing of said substance, an appliance corresponding in size and shape to said pattern and having an anterior and a posterior region; a closure section disposed in closing relationship with said open end of the primary section and having a cavity therein for retaining investment material which cooperates with said material in the chamber of the primary section to surround the appliance during formation of the latter, said open end of the primary section having a bevel presenting a fulcrum for said closure section; and hold-down means interconnecting said sections at said bevel to permit limited fulcruming of the closure section relative to the primary section about the axis of the fulcrum disposeD rearwardly of said posterior region of the appliance to thereby restrict expansion of said curable substance in the posterior region to a greater extent than in the anterior region during curing of the substance.
 15. The invention of claim 14, said hold-down means including an element extending from said closure section into said primary section and threadably received by the latter.
 16. The invention of claim 15, said axis intersecting said element.
 17. The invention of claim 14, said primary section having a sidewall defining the limits of said chamber and said open end defining the top of said sidewall, said bevel being disposed at said top of the sidewall and sloping downwardly therefrom to the outer margin of the sidewall. 